An organic electroluminescent (EL) display, being a kind of light-emitting apparatus, is a new type of flat panel display and formed from an array of organic light emitting diode devices (OLEDs). In general, an OLED device is comprised of an electrode serving as the anode, an electrode serving as the cathode and several thin organic layers sandwiched between these two electrodes. The organic layers include at least one emission layer. The emission layer is formed of a fluorescent organic compound, phosphorescent organic compound or a light emitting materials such as a quantum dot (QD) to emit light at each emission color. Applying voltage to the OLED device, holes and electrons are injected from the anode and the cathode, respectively, and form excitons in the emission layer. Then these excitons recombine and release their energy as emission of light. One of the tasks to be accomplished in development of such an organic light-emitting apparatus is improvement of the emission efficiency. The OLED device generally has such a structure that an anode, an organic layer including an emission layer, and a cathode are one-dimensionally stacked. At this time, the refractive index (approximately 1.7 to 1.9) of the emission layer is larger than the refractive index of air. Therefore, most of light emitted from the inside of the emission layer is totally reflected at an interface of the stack film at which a high refractive index changes to a low refractive index. The totally reflected light becomes guided-wave light propagating in a horizontal direction of a substrate, and then is confined inside the OLED device. The ratio of light which can be extracted for use to the outside (light extraction efficiency), of light emitted in an emission layer, is generally only approximately 20%.
Therefore, in order to improve the emission efficiency of the organic light-emitting apparatus, it is important to improve the light extraction efficiency. Among conventional technologies, for example, “Appl. Phys. Lett., 69, 1997 (1996)”, “Appl. Phys. Lett., 81, 3921 (2002)”, and “Appl. Phys. Lett., 88, 073517 (2006)” each describe that, when a cavity is introduced into the OLED device to make use of an interference enhancing effect, the light extraction efficiency can be improved. Further, “Appl. Phys. Lett., 88, 073517 (2006)” reports that an external quantum efficiency is improved by 35% by means of a cavity.
In addition to the conventional technologies described above, for example, Japanese Patent Application Laid-Open No. H11-283751 proposes a method involving providing a periodic structure (diffraction grating) in an upper or lower portion of an organic layer (on a light extraction side or on a side opposite thereto), in order to prevent total reflection to thereby suppress light confinement inside the OLED device.
However, even in the case of the structure provided with the cavity or the structure provided with the diffraction grating, the improvement of the light extraction efficiency is insufficient. Therefore, it is necessary to further improve the light extraction efficiency. However, up to now, extensive studies have not been conducted to strike a balance between a cavity which is required to have flatness in order to generate interference light and a periodic structure which is required to have unevenness in order to generate diffraction light, thereby further improving the light extraction efficiency.